I need some help with testing this electric VDO jet gauge tachometer. It is set up for 6 or 12 volt. It has a terminal for positive (1), negative (-), 6v (+6), and 12v (+12). Not sure how to bench test this, what wires go where. Some electrical guidance would be appreciated. Thanks

I think bench testing would be difficult. The way I see it you would either put 6 (+6) or 12 (+12) volt positive in their respective locations, only one, whatever you are using. (-) would be negative either 6 or 12 whatever you are using. (1) would be a negative input from the coil of the vehicle, where the points connect to on the coil. It is also labeled (1) on the coil. Positive is labeled (15). (Look at the wiring diagrams on the samba, all years). You would have to test it on a car. The negative from the coil pulses on and off when the points open and close. That is how it determines the RPM along with the selector switch for 2, 4, 6, 8,. That would be set to how many cylinders there are on the engine.

Last edited by pwmcguire on Tue Feb 11, 2014 10:31 pm; edited 1 time in total

If you have a battery charger you can test it/calibrate it without another vehicle/tach. Here is a link, go a little past half way down to the test section. The charger should make a tach set up for 4 cylinders read 3600 rpm. This works great, I've done it several times with converted factory techs and is very accurate.

The battery would go to the positive and negative terminals, the battery charger is used as a signal source.

Clip of the above link

PREFERRED SIGNAL SOURCE...an inexpensive 12v battery charger can be used as a deadly accurate signal source for the re-cal of your tach. This signal source is so steady and accurate that there is no need to compare the reading with another V-8 calibrated tach. Battery chargers typically use a rectifier bridge to convert the AC power source into DC. This turns the 60 hertz sine wave input into a 120 hertz ripple wave DC output. The ripple wave output is be read as 3600rpm by the stock rotary calibrated RX-7 tach. For a V-6 application, an RX-7 tach should be adjusted to read this signal as 2400rpm. For a V-8 application, adjust your RX-7 tach to read this ripple signal as 1800rpm. Please note that you must use a 12v battery for a power source, and the battery charger as a signal source only. The ground wires for both the battery and the battery charger should be connected to the ground terminal of the tach.